Conventional polysilicon gate stacks have become increasingly unsuitable due to excessive gate leakage as the gate dielectric is proportionally thinned as gate length is decreased. The introduction of novel gate stack materials including high-K (HK) dielectric materials and metal gates has enabled the continuation of Moore's Law.
In a replacement metal gate (RMG), or “gate last” process, the polysilicon gate is replaced with a metal gate stack. P-type field effect transistors (PFETs) and N-type field effect transistors (NFETs) utilize different metal stacks to achieve a desired work function, and thus enable a desired threshold voltage. The use of different metals in adjacent devices typically requires numerous, time-consuming steps to fabricate them. Therefore, it is desirable to have improvements in replacement metal gates and fabrication methods to reduce time and cost of fabrication.